The present invention relates to controlling charge accumulation on samples in charged particle beams systems and, in particular, to the neutralization of positive charge accumulation on samples in Focused Ion Beam (FIB) systems.
Insulating samples in charged particle beam systems tend to accumulate electrical charge because the charged particle beam brings charges to the sample and ejects charged secondary particles from the sample. The charge accumulating on an insulating sample can adversely affect the focusing and positioning of the charged particle beam and can inhibit the emission of secondary particles used to form an image of the sample or to analyze its composition. In a typical focused ion beam system, positively charged gallium ions impact the sample, knocking out secondary electrons and both positive and negative secondary ions. The net flow of charges typically leaves the sample positively charged.
Various methods have been used to eliminate unwanted charge accumulation. One method is to provide a conductive layer to drain the charge from an otherwise insulating sample. The conductive layer can be formed, for example, by depositing a thin film of a conductor such as gold or by implanting ions, such as gallium ions, into the sample.
Another method of eliminating accumulated charge is to neutralize the charge by delivering opposite charges to the sample. For example, an accumulation of positive charges can be neutralized by directing a beam of low energy electrons from an electron flood gun toward the sample. Such a neutralization system used with a focused ion beam (FIB) system is described, for example, in U.S. Pat. No. 4,639,301 for xe2x80x9cFocused Ion Beam Processingxe2x80x9d to Doherty et al., which is assigned to the assignee of the present application. Charge neutralization flood guns in FIB systems typically require an unobstructed line of sight from the flood gun to the sample surface and are traditionally mounted to the side of and above the sample. To provide an unobstructed line of sight from the electron gun to the sample, the final lens of the FIB column is spaced away from the sample. Increasing the distance between the sample and the final lens, referred to as the xe2x80x9cworking distance,xe2x80x9d reduces the ability of the ion optical column to focus the ions, thereby decreasing the system resolution.
Alternatively, an electron flood gun can be mounted below the final lens and substantially perpendicular to the primary ion beam. A negative electrical potential below the lens, is then required to deflect the neutralizing electrons towards the sample. This scheme also requires substantial spacing between the lens and sample resolution. Secondary particle detectors, which are routinely used with focused ion beam systems and which need to be close to the sample to collect a large portion of the secondary particles, also necessitate spacing the ion beam lens away from the sample, thereby increasing the working distance.
U.S. Pat. No. 4,818,872 to Parker et al. describes a system in which neutralizing electronsare directed from a flood gun through a deflector positioned below the final ion lens and then to a sample. The deflector requires that the final lens be positioned away from the sample, thereby increasing the working distance and reducing resolution.
Another problem with the electron flood gun described in the Doherty et al. patent is that operation of the flood gun interferes with the use of secondary electrons for imaging. The Doherty et al. patent describes alternating between the use of a neutralizing flood gun and secondary electron imaging because secondary electron imaging is apparently not practical when neutralizing electrons are being directed to the sample. When using electrons from a flood gun for neutralization, it is known to use secondary ions, instead of secondary electrons, to form an image of a sample. The signal-to-noise ratio for an image produced from secondary ions, however, is typically lower than the ratio for an image produced from secondary electrons because there are fewer secondary ions than secondary electrons. In today""s microfabrication environment, engineers and scientists need a good signal-to-noise ratio to create high resolution images of extremely small features.
Another method of charge neutralization is described in U.S. Pat. No. 4,748,325 to Slodzian for a xe2x80x9cMethod and Device to Discharge Samples of Insulating Material During Ion Analysis.xe2x80x9d In the method described by Slodzian, a primary beam of ions sputters secondary ions that are accelerated towards a detector by an acceleration electrode. Some of the secondary ions strike the acceleration electrode and cause electrons to be ejected. The electrons are then directed back to the ion beam impact area of the sample by a charged, conductive film surrounding the ion beam impact area. Deposition of the conductive film and conductors to charge requires an extra processing step and can destroy the sample.
Thus, there remains a need for a charge neutralization technique that does not involve a deposition step, which takes time and alters the sample, does not require increased ion lens working distance, and allows for secondary electron imaging.
An object of the invention is to neutralize charges on a sample upon which a charged particle beam is directed.
The present invention comprises an electron source that provides electrons that are directed through the final lens of the ion optical column to neutralize at least a portion of the accumulated charge on the sample. The invention can optionally be combined with collection of secondary electrons through the final ion lens.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.